1. Field of the Invention
The present invention relates to a method and device for measuring laser power emitted by the distal tip of a light conducting catheter and more particularly to an improved integrating sphere for measuring energy radiated either from a point or a diffuse source of light.
2. Prior Art
A number of surgical techniques employing laser radiation have been developed. For example, laser devices have been employed as surgical scalpels. Laser photo-coagulating devices are employed in laparoscopic surgery to effect coagulation during surgery. Laser catheters are inserted into blood vessels for the ablation of atherosclerotic plaque. Such techniques have created a need for medical laser systems having variable wavelength and precise power output levels for performing these various procedures.
Photodynamic therapy (PDT) is a procedure involving photoirradiation of tissues having photosensitive molecules concentrated therein. Very early in the development of clinical photodynamic therapy it was found advantageous to use diffusing type light delivery systems to achieve the delivery of more uniform illumination to a volume of tissue. Use of fibers with diffuser tips gave rise to the problem of how to measure the total light emanating from the diffuser tip. Clearly, techniques using standard flat surface power detectors are inadequate for measuring total power from a diffuser tip. At present, the use of an integrating sphere appears to be the most efficient and accurate method of measuring the power delivered by diffuser tips to surrounding tissue. A number of problems arise through the use of prior art integrating spheres for measuring the power output of a surgical optical fiber:
a. the sterility of the fiber must be maintained while performing the power measurement; and PA1 b. the light emitting tip of the fiber should advantageously be placed in a fluid environment while performing the power reading to simulate index matching with tissue; and PA1 c. in addition to the foregoing problems with prior art integrating spheres, it is frequently desirable to determine whether the wavelength of the light being measured by the integrating sphere is within a specified band.
In PDT it is particularly necessary to precisely control the amount of laser radiation delivered to biological tissues during the photo-therapeutic procedure. The appropriate amount of radiation to produce a therapeutic effect is known to vary with the amount of photosensitive molecule taken up by the tissue and the irradiation technique employed. In addition, the wavelength of the irradiating light must be carefully controlled. If the power level and wavelength of the illuminating light are carefully controlled, this reduces the number of variables that must be determined to achieve accurate dosimetry.
The difficulty of controlling the amount of laser radiation delivered to tissues is aggravated when various peripheral devices such as fibers of different diameter, having varying optical properties and power requirements are used in the same system. Moreover, the optical properties and power output requirements of a particular optical fiber tip may gradually change due to wear, debris build-up, etc., requiring recalibration.
Hertzman, in U.S. Pat. No. 4,580,557, describes some of the problems associated with monitoring the power output of surgical laser accessories. To overcome these problems Hertzman describes a surgical laser system which includes a laser, interchangeable peripheral output devices, a sensor effective to sense the power output of a particular peripheral device, and a control circuit for calibrating the radiation output of each output device and interlocking the system to prevent use of a peripheral output device before it has been calibrated.
The "calibration pod" or "sensor" described by Hertzman is provided to calibrate the peripheral surgical devices which are selectively attached to the system. The calibration sensor consists of an integrating sphere having a first aperture through which the peripheral surgical device (optical fiber) may be inserted or its output beam directed, and a light sensitive electronic device such as a light sensitive silicon diode located in a wall of the sphere. The inside surface of the sphere is a diffusing surface such as sand-blasted metal, anodized aluminum or magnesium oxide or sulfate as is well known in the art. The use of a planar baffle between the first (source) aperture and the detector to prevent the direct illumination of the detector by the source is also well known in the art. Hertzman reports that at any point on the inner surface of the integrating sphere, the amount of illumination is essentially constant and insensitive to the exact positioning of the peripheral surgical device with respect to the sensor.
As mentioned earlier, in photodynamic therapy, or possibly other medical application of lasers using dispersing type delivery system, it is necessary to be able to measure the total power/energy out of an optical fiber delivery probe which energy is radiated into a very large angle, as for example, by diffuser or dispersion type probes. Standard light power/energy meters for lasers are based on flat surface detectors (thermopiles, diodes) that require the light from the laser to be coupled to the detector with a small angle of incidence. This is generally acceptable when working with the laser beam or the output of a flat polished optical fiber of reasonable numerical aperture, &lt;0.5 N.A. When using delivery systems with diffusing type tips (e.g. cylindrical or spherical) or tapered contact tips (i.e. surgical contact fibers), the light is emitted from the fiber tip in the forward, sideways and backward direction making an accurate measurement of the total output of the delivery system not possible with standard flat surface power/energy meters.
In summary, prior art integrating spheres do not have a source aperture adapted to accommodate an optical fiber tip immersed in a fluid medium to simulate the tissue environment prior to measuring the energy. In addition, prior art integrating spheres have no provision for maintaining sterility of the fiber tip that is the subject of the measurement. Prior art integrating spheres lack the capability of measuring the wavelength of light present in the sphere and have poorly designed light baffles which do not maximize the number of light reflections occurring in the sphere prior to detection. It is desirable to maximize the number of reflections to give the most accurate energy measurement.